Seals for static and dynamic applications eventually wear out. Rotary lip-type seals are generally made from substantially soft materials such as elastomeric materials and certain types of soft plastic materials like polytetrafluoroethylene (PTFE). These materials adjust readily to variations in tolerances and in so doing vary the areas of contact between the seal and the sealing surfaces. In dynamic applications, an increase in the area of contact between the seal and the sealing surface under high sealing stress results in a temperature increase, which causes a decrease in the tensile and shear strength of the softer seal material resulting in a decrease in seal life. The greater the area of contact, the higher the temperature at the sealing area and the faster the seal will likely fail.
Many high temperature seal materials have a higher modulus of elasticity with a higher hardness. The higher hardness reduces flexibility, making it more difficult to affect sealing. These materials thus generally make poor lip-type seals. In order to achieve good sealing, higher modulus materials generally require higher sealing stress than low modulus materials. Furthermore, in order to achieve long seal life, the appropriate sealing stress determined by the viscosity of the media and the dynamic sealing temperature should be known. Since hard plastic materials that can withstand elevated temperatures generally have very low elongation, the range in tolerances of sealing stress under which they can function properly is limited.